"They have a pretty good idea of who it may be," a U.S. official said. But the official cautioned that "it's not 100% yet." Neither official, who spoke to CNN on the condition of anonymity, was willing to identify the suspect, citing the ongoing investigation.

The official said, "We've been using all means to identify the person." That includes voice analysis and analyzing metadata from the video as well as other methods, according to the second U.S. official. The official declined to elaborate on what those other methods were.

In late August, British Ambassador Peter Westmacott said his country was close to identifying the ISIS militant. Westmacott didn't elaborate about the identity in the August 26 interview, which aired on CNN's "State of the Union" with Candy Crowley.

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"We're putting a great deal into the search," Westmacott said at the time, referring to the use of sophisticated technology to analyze the man's voice.

The subject is believed tied to a group of extremists based in London, the officials said. FBI agents have been working with British authorities to nail down who the subject is, a priority for both governments.

The same U.S. official said the United Kingdom has investigative interest because the masked man is believed to be a British citizen who traveled overseas to engage in conflict. The United States has investigative interest because he killed an American.

Authorities are still trying to determine the identity of the militant speaking in the video of American journalist Steven Sotloff and determine if it is the same person in the Foley video, according to one of the officials. The source said "it's too premature" to know with any certainty just one week since the video of Sotloff's death was published.

The source said British and American authorities would work together to find the masked man, who is believed to be in Syria.

"This person is in an environment that's highly dangerous. He could meet his demise on the battlefield," the official said, though the sources did not rule out bringing the suspect back for trial in British or American court.

Foley disappeared on November 22, 2012, in northwest Syria, near the border with Turkey. At the time of his disappearance, he was working for the U.S.-based online news outlet GlobalPost. The video of his beheading was posted to YouTube on August 19. Sotloff disappeared while reporting from Syria in August 2013. The video of his death was posted September 2.